The invention relates to stretch wrapping apparatus having prestretch rollers which stretch packaging material before wrapping a load.
Stretch wrapping is a packaging technique which dispenses and wraps a web of stretch wrap packaging material in a stretched condition around the load to cover and contain the load.
Early stretch wrapping techniques positioned the load on a turntable and fastened the leading end of the web of stretch wrap packaging material to the load or the turntable. As the turntable rotated, the roll of stretch film was restrained by a brake to stretch the packaging material in the direction in which it was being dispensed as the packaging material was wrapped around the load. A similar stretch wrapping technique revolved a film web dispenser around a stationary load to wrap the stretched film around the load.
As the field of stretch wrapping developed, Lantech invented a stretch wrapping apparatus which employed a film web dispenser having two closely spaced rollers to stretch a web of thermoplastic stretch film packaging material. Such rollers are known as prestretch rollers. The downstream prestretch roller has a faster surface speed than the upstream prestretch roller. This stretches the packaging material between the upstream and downstream prestretch rollers in the dispensing direction.
These closely spaced prestretch rollers were driven by the demand for film web by the load and were sometimes assisted by an electrical motor. In one arrangement, the two prestretch rollers included sprockets which were connected by an endless chain to cause the downstream prestretch roller to have a faster surface speed than the upstream roller and thereby stretch the web of stretch film over its yield point in the dispensing direction.
It is preferable that a stretch wrapping apparatus with prestretch rollers have the following features. The upstream and downstream prestretch rollers should be closely spaced during operation to prevent neckdown of the film web as it is being substantially stretched in the dispensing direction. Since the forces exerted on a wide web of stretch film between the prestretch rollers is very significant, in the order of 50 pounds of force, the frame and the mounting of the prestretch rollers should be required to be very sturdy. The surface of the prestretch rollers should be covered by the stretch film over a sufficient extent such that the stretch film does not slip on the prestretch rollers. The rotational inertia of the prestretch rollers should not be so high as to require excessively high power to vary their velocity.
To accommodate these requirements, a series of idler rollers are used to support the stretch film proximate to the prestretch rollers and maintain the tack side of the film web facing the prestretch rollers. As a result, the film web follows a tortuous path around the closely spaced prestretch and idler rollers.
Before operating a stretch wrapping apparatus having prestretch rollers, it is necessary to thread the stretch film along the tortuous path. This threading operation is difficult and time consuming because due to the close spacing of the rollers, an operator cannot manually carry the stretch wrap packaging material between the rollers. Rather, the operator must feed the stretch wrap packaging material from one side of each roller nip with one hand and pull the stretch film from the other side of the roller nip with the other hand. This procedure is rendered considerably more difficult because of the large widths of the stretch wrap packaging material which are used commercially. Such webs generally have a width in the range of 10 to 30 inches. Other problems in threading resulted from the fact that the operator was required to place his fingers proximate to the nip of the rollers and blindly feed the film from the rear of the frame which carried the rollers, which made the threading operation difficult and more time consuming.
Lantech addressed the threading problem in one of its earlier patents, U.S. Pat. No. 4,747,254, which is incorporated herein by reference. This earlier arrangement reduced the difficulty of threading a web along a tortuous path by using a retractable leader which ran generally parallel to the path of the film web around the rollers to lead the film along the path. However, this arrangement required the leading end of the film web to be attached to the leader and the leader to be slowly pulled through the tortuous path while rotating the prestretch rollers, releasing the film web from the leader and retracting the leader.
To overcome a number of these difficulties, Lantech designed and commercialized a stretch wrapping apparatus which is disclosed in U.S. patent application Ser. No. 07/611,114 now abandoned, which is incorporated herein by reference. This apparatus employed a first dispenser frame assembly for supporting the upstream and downstream prestretch rollers, and a second dispenser frame assembly for supporting the idler rollers. The frame assembly for supporting the idler rollers could be pivoted away from the frame assembly for supporting the upstream and downstream rollers so as to provide an open loading position in which the stretch wrap packaging material could be easily carried between the idler rollers and the prestretch rollers, and a closed operating position in which the idler rollers were proximate to but spaced from the upstream and downstream prestretch rollers.